Nodular iron process of manufacture



Oct. 7, 1958 T. w. cURRY NODULAR IRON PROCESS 0F MANUFACTURE 5Sheets-Sheet 1 Filed Feb. 4, 195'?` Oct. 7, 1958 v T. w. cURRY NODULARIRON PROCESS OF MANUFAOTURE 5 Sheets-Sheet 2 Filed. Feb. 4, 1957 Oct. 7,1958 T. w. CURRY NODULAR IRON PROCESS oF MANUFACTURE 5 Sheets-Sheet I5Filed Feb. 4, 1957 (IOOX NITAL ETCHED) PRESENT CAST IRON PRODUCT (IOOXUNETCHED) REGULAR SUPER DE LAVAUD UNIFORM DISTRIBUTION OF FEWER, LARGERGRAPHITE PARTICLES TYPICAL SIZE, PATTERN, 8l DISTRIBUTION OF GRAPHITE.

WHICH TEND TO BE SPHERICAL.

(5OOX NITAL ETCHED) PRESENT CAST IRON PRODUCT JVVENTOH THOMAS W OUR@ YBY Simms* L Sme; ATTP/VEYS' REGULAR SUPER DE LAVAUD process.

United States Patent Oice u 2,855,336 Patented Oct. 7,., 19758 2,855,336NODULAR IRON PROCESS OF MANUFACTURE Thomas W. Curry, Lynchburg, Va.

Application February 4, 1957, Serial No. 637,957

13 Claims. (Cl. 148-3) The present invention relates to the productionof nodular iron and more especially to an iron which is cementitic uponchill casting and which is thereafter converted to a nodular iron havinggraphitic nodules, graphitic aggregates and often includes graphiticflakes of short length. The instant application is closely related to mycopending patent application Serial No. 549,322, entitled Nodular CastIron and Process of Manufacture Thereof, tiled November 28, 1955, andpatent application Serial No. 638,118, Cast Iron Product of Manufacture,liled February 4, 1957..v

.The product of the present invention being particularly though notexclusively manufactured for use in De Lavaud pressure pipe, must becompetitive in cost with regular grey iron pipe, yet its properties bothphysical and mechanical must be superior to the present grey iron pipecastings, particularly in impact and shock resistance. The improvedphysical properties of the nodular iron such as is manufacturedhereunder will materially reduce failures previously encountered withthe relatively brittle greyiron pressure pipe; breakages being caused intransportation and in ditch installation, earth movement and beamloading in the ground of the length of pipe or breakage in an assemblyof pipes as may be caused by surface traliic and improper support of thepipe with till. Nodularl iron of prior manufactures, as for instancethat iron having a residual of magnesium above 0.01%, manganese in therange of 0.35% to 0.60%, relatively low copper, chromium and molybdenumand other tramp elements, though having excellent physical propertiesfor making pressure pipe, has not as yet been extensively used becauseof the residual amounts of carbide stabilizing elements and their effectupon the annealing cycle. Available ferrous materials have too highcontent of these carbide stabilizing elements to make satisfactorynodular iron pipe with the equipment now in use in the De Lavaud A veryexpensive installation for annealing would be required to producesatisfactory pressure pipe having too high a content of carbidestabilizer in the nodular iron. Because the higher content of thesestabilizers materially lengthens the annealing cycle and the length ofthe oven chain must increase as the cycle time increases, it isquestionable whether it is mechanically practical to construct thenecessary length of oven chain with suliicient strength at the highannealing temperature (l750 F.) to operate under the productionrequirements of the De Lavaud process. An alternative would be multipleovens at greater capital expenditure, and if the additional cost Wereacceptable the higher magnesium content of the conventional method ofmaking ductile iron would prevent annealing in the present short cycleequipment. Today it is believed that to obtain the base metal with acarbide stabilizer content suitable for making nodular iron pressurepipe centrifugally by the De Lavaud process would increase the metalcost per ton of pipe $10.00 over the cost of the metal for conventionalygrey iron pressure pipe. In prior art developments of inventors Milliset al., lhrig, Chrome and Morrogh comparatively great quantities ofexpensive magnesium and cerium alloys are required in the manufacture ofthe nodular iron described in their patents. Among the more outstandingpatents in this particular field are the following: 2,485,760, Millis etal.; 2,750,284, Ihrig; 2,747,990, Morroghg' 2,692,196, Hulme; andBelgian Patent No. 514,115 of Crockett and Hulme. With respect to theproduct of Millis and Gagnebin the starting material must be a pig ironwith low manganese content or a mixture of steel scrap and pig ironwherein the latter is of at least 50% of the total volume. As to theproduct of Morrogh, there is required a relatively high residum ofcerium therein to overcome `the electof inhibiting-elements inproduction. The product of Ihrig is made by using expensive special pigiron and/or scraps having a relatively low kmanganese content. Otherwisecostly chemical refinements would be required during melting. In Ihrigsprocess the cold additions made as ladle treatments are in such highamounts that it'would be questionable Whether his process `could be usedwithout some means of reheating during or after inoculation treatment.The present process yielding the iron product desired calls for the useof an inexpensive steel scrap melt having a relatively low phosphoruscontent and relatively high content of manganese, namely 0.40 to 0.60%.Moreover, in the manufacture of the particular nodular iron underconsideration, it must be adapted to unmodified casting machines andannealing equipment, such as are normally used in industry. The presentproduct is transformed from a cementite on casting to a structure havingpearlite, ferrite, graphite, in various shapes including: nodular, flakeand aggregate, and a relatively very small amount of cementite, theaggregate being similar to temper carbon aggregate in malleable iron.Thus, the product in question may be called a nodular iron having anabsence of a dendritic pattern. The structure of the nodular iron metalproduced herein and the shape of the graphite aggregates or nodules aresuch that the metal has measurable ductility; the mechanical propertiesof same have been improved by an increase in the ultimate tensilestrength, a denite yield point, and measurable elongation. Naturally,ductility improvement accounts for greater resistance of the metal toimpact or shock loading. i

Nodular iron made by known processes is often of a gradewhich could notbe used commercially in De Lavaud casting machine processes, because ofdeficiencies in the controls. By controls is meant diverse means foreliminating, nullifying or inhibiting the deleterious effect of trampelements sometimes called subversives or contaminants., Where thecontrols used in the manufacture of certain nodular irons were such asto give good results, the expense of the control methods made theoperations commercially unsatisfactory, considering many of thecommercially available starting materials and additives, I have devisedthe process to yield a-satisfactory product wherein all the controls aredelicately and accurately employed in such a way as to give universallygood results, and Where the expense of the combination of steps is suchas to be economically competitive with related processes which areemployed in the manufacture of grey In the drawings: Y Fig. 1 is a owsheet of myprocess, basic cupola;

Fig. 2 is a iloiiv sheet f myprocess, acid cupola;

Fig'. 3 is a'photomicrogr'aph at magnification 100 "of the iron metal ina conventional "super De Lavaud pipe showing the type, size, pattern anddistribution of graphite il'akes disposed liii dendwwpa'tten;

Y Fig. llis a phb'tomid'graph'at magnification 100 "of the iron metal ina pipe made 'by my process. The product lshows no "tendency 'to 'theVdendritic pattern of Fig. 3 and th'e'pr'oduct is iron containinggraphite nodules, aggregates and ake's;

Fig. 5 `is photmic'rograph vat magnification 500 of the iron metal in anotal etched specimen taken from conventional super De Lavaud pipe;

n Fig. 6 is a photornicrograph :at magnification 500 of the iron metalin a nital etched "specirnen'taken from pipe made by my proess.Graphitic nodules, akes and aggregate's'are shown in a'm'at'riX ofsubstantially entirely ferrite, vtraces of pearlite and c'einentitebeing present.

It will be vnotedtlatin the `De Lavaud process, pipe is centrifugallych'ill'cast in steel molds, the pipe is thereafter annealed, and it isthis process which is particularly applicable though not exclusivein thepresent instance of manufacture. Wall thickness in my nodular iron pipecan be-ma'de thinner than in v'conventional grey iron pressure pipe,andyet my pipe `will have the same or greater strength. The ductility ofthe metal in this pipe has been'found considerably Igreater than suchgrey iron pipe, and it is less subjected to rupture through shock. Thefollowing chart, for example, is a comparison of of the mechanicalproperties of product pipe made by the De Lavaud steel mold castingprocess, where super De Lavaud is conventionally produced, and on theother hand a nodular iron pipe of thesame diameter and less lwallthickness as made by both of my processes, yet similarly cast in a DeLavaud centrifugal steel mold. Both process pipes have been annealedafter casting. Note that in the case of the former super De Lavaud thewall thickness was 0.42"'and in the present nodular iron pipe the wallthicknes'swas 0.25 to 0.35.

DE -LAVAUD PROCESS were rough and unserviceable. Such expensive pipe hadadhering non-"metallic inclusions. In these prior art processes,moreover, the fluidity of the metal was such that best results wereobtained by increasing the wall thickness quite beyond that required forthe standard grey iron pipe. It was also found that nodular iron made bystill other processes investigated required a higher pouring temperatureat the expense of steel mold life, to produce a useable but notcommercially satisfactory surface on the inside and outside of the pipe.

In order to make -good nodular iron, for instance, such as can be usedin pipe with advantage, it is quite necessary to have flexible controlsthroughout the process. For example, some of the constituents which mustbe controlled within desirable limits are percentages of carbon,sulphur, manganese, silicon and phosphorous. In general, the ferrousmaterials included within the cupola charge comprise cast iron and steelscrap and returns from the nodular pipe casting process. As al practicalmatter, it is desirable when charging steel scrap to eliminate from thescrap, before melting, armatures of electric motors because of thepresence of copper, to eliminate rubber cov'eredsteel because of highsulphur content and also to eliminate aluminum, bearing and othernon-ferrous metals. Steel scrap containing these inhibiting elementsadversely affects lthe desired formation of spheroidal graphite, for avery small percent of these inhibiting elements in the presence ofcopper and titanium will upset the process and prevent the formation ofgraphitic nodules and compacted graphite during annealing. Due tovariations in the chemistry of the ferrous charging materials, controltreatments 'at various stages of the process must be undertaken to bringpercentages of the desired constituents and other elements within apreferred range. These control treatments will be detailed hereinafter.

PROCESSES In general my process comprises a melting stage in 1"Keu-eaoOu-saoeo. 1" Kee150,000-65,000.

`Pipe Wall Thickness, inches 0.25-0.35.

1 Full length of pipe (157%").

It will be observed from theabove that the present pipe has burstingtensile strength of from 21/2 to 3 times that of grey iron pipe whichhas a wall 20% thicker than the present pipe.

By Way of explanation, if the ductility and ultimate strength of a metalis increasing, the modulus of elasticity will increase. In testing allmechanical properties of pipe, as above the Cast Iron Pipe Associationuses the secant modulus of elasticity and therefore the lower the valuefor MB, the better the 'mechanical properties of the pipe. It isapparent from the above chart that the design of the present nodulariron pipe with its better mechanical properties could be modified,particularly its wall thickness, and thus perform much moresatisfactorily than conventional grey iron pipe. This reduction in thewall thickness of the pipe would also naturally result in cost savings.

Prior tests of De Lavaud nodular iron made with processes other than thepresent have vcreated a casting problem in'that the insiderand'outside-surfacesfof thepipe which the ferrous charging 'materialsare melted in acid or basic cupolas, followed by a calcium carbideinjection 'desulphurization Sometimes in practice the desulphurizationstep may be omitted for often available materials and chemical controlin basic melting bring the sulphur content below the desirable maximum.In practice this desulphurization step is normally followed 'by a basicelectric furnace re-heating step in which carbon and silicon areadjusted. Of course this third step may be eliminated dependent uponcharge and cupola construction. The ensuing step is a treatment step toconvert the molten material'from typical grey iron characteristics tonodular iron. Following this treatment step is a step of linoculation tomake final adjustment of the elements present in the'molten mass. Fromthis inoculation step, the materials pass to a chill casting step whichis followed by annealing.

Referring tothe drawings in Figures 1 and-2, I have diagrammaticallyVshown `preferred processes'for carrying 5 out my invention. Alternativeprocesses will be apparent from the following description.

Step I (basic and acid) In a typical example, Figure 1, there is chargedinto a basic cupola in the melting step, No. I, steel scrap and `castiron, the latter including returns, from the nodular pipe together withcoke and flux to -form a basic slag. The following is illustrative:

CaO.MgO. SiO2.Al2O8 factor is greater than 1.0. The foregoing comprisesthe material which is to be charged. All this material is now chargedinto the cupola and air is injected under pressure for combustion of thecoke and to obtain a molten material, the range of temperature being2850- 2950 F. This molten metal is tapped into either a forehearth orladle, as required. During melting, iron oxide and manganese oxide areformed, the latter being higher in total content than the former named,about 1.5% in the slag. As previously indicated the low sulphur contentin the basic cupola method often makes desulphurization unnecessary. Acooling of molten mass occurs and it leaves the forehearth or ladle, atabout 2600-2700 F.

A typical charge in the basic cupola of Step I is as follows:

(b) Coke and Flux Per Ton of Metal Charged Into Basic Cupola:

150# Stone (Mixture of limestone and dolomite). 350# Coke.

1 Silicon loss in cupola is calculated at 33%.,

2 Total carbon.

The percentages of steel scrap and nodular or ductile iron returns shownin the cupola charge may be varied depending upon shop circumstances.Steel scrap would be in the range of 50% to 100% of the charge withnodular iron pipe returns, scrap or cast scrap', making up the balance.The silicon content of the metal out of the cupola would be in the rangeof from 0.30% to 1.40%.

ACID CUPOLA METAL CHARGE A typical acid cupola example on the other handis illustrated in Figure 2. Inthis process the melt includes 2.90-3.50%total carbon and approximately 0.04-0.15% sulphur. The metal chargeincludes approximately 85% steel scrap and 15% nodular or ductile ironcast returns. Coke is introduced at from l2 to 18% of the total volumeand the flux includes approximately 2% dolomite. The acid slag wouldhave a CaO.MgO Al2O3.SO2

factor of less than 1. The melt leaves the acid cupola at approximately2850 F. and because of its relatively high sulphur content, must besubjected to desulphurization as in the following step.

Stepl Il (acid only) Inl Step II of the drawings of Figure 2, the acidmelt isdesulphurizedl in Ladle,` or forehearth, byinjection of calciumcarbide carried in an inert carrier gas such as argon, carbon dioxide ornitrogen. A typical injection to desulphurize acid cupola iron from0.05% to 0.02% would be as vfollows: 10 lbs. calcium carbide in 7 cubicfeet nitrogen gas injected into a ton of metal according to thefollowing.

Calcium carbide is injected at minus 4.5 pounds -pen minute with a gasow rate of minus cubic feet per hour and the injection nozzle is placedat 12-15 inches below the surface of the metal bath; nitrogen gaspressure at the tip of injection tube is at 15 pounds per square inchpressure. i

Excesses of calcium carbide or too rapid a feed rate create calciumcarbide losses through no metal-wetting of the calcium carbide with aconsequent lack of reaction and quite a loss of temperature in the melt.Under the above conditions l0 pounds of injected carbide will remove onepound of sulphur.

The 10W sulphur content carrying over through Step III, is essential forthe successful functioning of subsequent Step IV. As stated, the moltenmetal as taken from the either basic -or acid cupola (Step I) is at atemperature of from 2850-2950 F. The metal from the basic cupola howeveroften requires sulphur and carbon adjustment, hence it must be reheated.If the sulphur content is at 0.02% maximum and the total carbon at fromS20-3.70% adjustment and reheating may be eliminated.

Step Il (basic) Step II in the basic cupola diagrammatic drawings ofFig. 1 shows an electric furnace where the molten mass is reheated toraise its temperature, and percentages of carbon and silicon areadjusted, as is shown below. The metal charged into the basic electricfurnace in Step II has cold steel scrap and 50% ferro-silicon added forthe purpose. The temperature of the metal after these Step II additionshave been made is in the range of 2500- 2600 F., thus it is now raisedto 2850-2900 F.

The following is typical of the operating conditions of Step II:

METAL CHARGE TO ELECTRIC FURNACE 1825# cupola metal special steel scrap(cold) 90# 50% FeSi (calculated at 48% S content and 10% Si loss) Theadditions made to the molten mass in Step II of Figure 1 can be varieddependent upon the carbon and silicon content of the metal as it istapped from theladle (Step I). The cold steep scrap addition may varyfrom 0% to 20%, and the addition of 50% ferro-silicon will vary between40 lbs. and 100 lbs. per ton of metal charged.

The sulphur content of the metal tapped from the cupola may be in therange of from 0.01 to 0.05%, however, under normal operating conditions,it is expected that the sulphur content of this metal will not exceed0.02% maximum. Nevertheless, where the sulphur content of the metallfrom the cupola is in excess of 0.02%, then desulphurization, Step II,is utilized. Of course, metal with a sulphur content less than 0.02%, asin most basic operations, will normally proceed directly from the cupolato the electric furnace since Step II desulphurization is unnecessary.

The Figure 1 example shows the use of a 50% ferrosilicon alloy additionbut the process will work equally well if any commercial grade offerro-silicon is substituted for the 50% grade. By 50% ferro-silicon ismeant 50% silicon in the iron alloy. Commercial grades that,

f7 can be used are the commercial grades that run 30% silicon'in aferrous alloy, to,- 90%- silicon therein.

Step Ill ('b`a`sl'c andtzcid) `In the next step, Step IV (see Figures land 2), the metal is passed to a treatment ladle where nodularizingmaterials are-injected. The carbon and sulphur adjusted metal is tappedfrom the electric furnace of Step II (Figure 1 or desulphurizationforehearth Step II, Figure 2) in amounts suitable for handling in the DeLavaud pipe shop. As each tap of metal is made, an immediate injectionof calcium carbide and nodularizing agent is made in the transfer ladle.Inl the case of the acid cupola process the mechanical mixture ispressure injected simultaneously as the metal is deposited in the ladle.Treatment of the molten iron at 2850-2900 F. as it enters thenodularizing treatment ladle, Step II, changes the composition lof theiron so that graphitic nodules will form when annealed (Step VI) ifinoculation (Step IV) is made as it is poured into the pipe machineladle (Step V) as hereinafter explained. The molten mass leaves bothnodularizing treatment ladles at about 2650 F.

The following is an example showing a typical Step II:

(a) SIMULTANEOUS INJECTION F NCDULARIZING ALLOY AND CALCIUM CARBIDE PERTCN 6.4# alloyed mixture 0f Mg, Ce, Si alloy 14.4# calcium carbide 10cubic feet nitrogen carrier gas Si SlMn FITC Analysis 2. 39

To obtain residual magnesium in the range of 0.005 to 0.01% its presencein the nodular iron should be in excess of cerium by a minimum of fourtimes, to yield the best results with white iron pipe, through thenodular process herein. The nodularizing alloy itself contains a greaterexcess of magnesium, of course. Pure magnesium metal in this treatmentstep is unsatisfactory because it promotes a more stable cementite andinhibits transformation, a factor which would make the processill-adapted to the short cycle annealing (Step VI) for which the processis designed.

The silicon content in Step IV shown in the example can be in the rangeof 2.00% to 2.70%; the total carbon content could be in the range of3.30 to 3.70%; sulphur is at 0.02% max. and the process would besatisfactory.

The nodularizing alloy which is injected simultaneously with the calciumcarbide can be a magnesium-ceriurn-silicon ferro alloy. A typicalinjection as indicated in the figures given above comprises asimultaneous injection of 14.4 lb./T calcium carbide and 6.4 lb./T ofmagnesiumcerium-silicon ferro alloy in 10 cu. ft. of nitrogen carriergas.l This is equivalent to 0.025% magnesium. It will be appreciatedthat in recovering such critical quantities of residual magnesium(0.0050.01%) in the metal under these conditions, a control againstconsistent magnesium oxidation must be effected during injection. Forthis reason, I have selected the above control ratio of calcium carbideto magnesium, great care being taken to inject the alloy and carbide atprecisely the same moment. A'

typical nodularizing alloy is as follows: Mg 8.5%, Ce 0.5%, Si 45%, Al1.25% with the balance iron.

I have found that the residual cerium content of the nodular iron withthe above-mentioned alloy is suicient to develop the graphite structurenecessary for the improved physical properties of the metal and theimproved mechanical properties of the pipe. The amount of residualcerium in the metal is so small, however, that its presence can only heshown by the spectrograph and not correctly analyzed quantitatively.

Aside from the iron-silicon-cerium-magnesium group these alloys havebeen found suitable:

(a) Mg 15.0--20.0%, Ni S50-80.0%; and (-b-) Mg L20-15.0%, Si 30.0%, Ni40.0-50.0%.

Copper and nickel retard secondk stage annealing and thus when thesecopper andV nickel alloys are used in either acid or basic process, Iadd Mischmetal having 50% cerium added to offset the effect ofinhibiting agents present.

In this nodularization-'werecover from between 0.005% to 0.009%magnesium in the annealed product. Otherwise stated, there is a 25%recovery of residual magnesium considering the amount of magnesium alloyadded. Residual magnesium in excess of 0.01% does not produce thestructure desired in the pipe with the annealing cycle employed in theprocess. Residual magnesium from 0.005-0.009% will develop the desiredproperties. It has been found that the higher the magnesium content ofthe alloy, the more erratic is the control of residual magnesium in thenodular iron; moreover a residual of greater than 0.01% adverselyaffects the conventional annealing cycle that is so important in themanufacture of inexpensive castings.

It is calculated that an addition of cerium, a nodularizing alloy orMischmetal or both to place cerium residual is estimated in the range of0.001 to 0.003%.

In the present process the preferred nodularizing magnesium alloy is aniron-silicon-cerium alloy.

Many magnesium-cerium-silicon ferro alloys are susceptible ofsatisfactory employment in my process. Economics, of course, will bealfected with major changes in the cerium and magnesium contents ofthese nodularizing alloys. InV general, alloys, the composition of whichwill fall within the limits expressed below in percentages by weight,may be used:

Metal melted in a cupola is known to have oxides which somelinvestigators have described as slimes. These oxides are determimentalto ultimate physical properties of the iron and have an effect upon itsstructure formation. It is also known that elemental nitrogen and/ornitrogen compounds are detrimental to the physical properties/of' steeland 'grey iron. In this connection, Work has been done to show likewisethat hydrogen, generated in the 5000 F. arc of the indirect-arc electricfurnace has a detrimental effect upon the physical properties of greyiron.

In Step III of the process Where calcium carbide is' injected into themetal with nitrogen gas (N2), I believe that oxide slimes, nitrogen (N),nitrogen compounds and hydrogen may be removed from the metal and thisremoval has a desirable effect upon the annealing characteristics of thenodular iron. This concept is supported by recent efforts in Germany inwhich the melt is ushed with nitrogen gas to remove such undesirablenitrogen (N) and nitrogen compounds.

It is to be understood that whereas there is shown but one treatmentstation for injection at Step III and in the forehearth or transferladle of Step I, a plurality of injections may be used, for in practicethe greater the number of injectors, the greater the efficiency inoperation.

After the metal has been treated by the injection of the calcium carbideand the nodularizing agent in the nodularizing treatment ladle, it istransferred to the De Lavaud pipe machine Hoor. Generally, the metal isheld less than ten minutes in the nodularizing treatment ladle.

Step IV Step IV, which takes place when the metal is transferred to thepipe machine ladle is one in which a ferrosilicon addition is made toraise the silicon content and inoculate the molten mass immediatelypreceding casting (Step V), to promote formation "of a structure havingasses-,sse

the best mechanical properties after annealing (Step VI). The followingis typical of this inoculation treatment:

Composition of Metal Ferro-Silicon Addition at De Lavaud Machine Ladle 1Si S Mn P TO 5# 75% grade ferrosilieon alloy for ladle addition, perTon. Analysis 2. 54 0. 015 0. 45 0.10 3.50

1 At 75% Si and 80% recovery, this addition adds 0.15% Si to the metal.

In Step V, Figures I and II, the pipes are chill cast in the De Lavaudmachine, yielding cementite. The process of casting an iron on a chillrefines the grain size, and all other factors remaining equal, thesmaller the grain size of the white iron, the more rapidly the desiredtransformation from cementite to pearlite and ferrite takes place duringannealing. To illustrate the transformation that takes place a 0.008%residual Mg nodular iron, chilled cast and annealed, would have the samephysical properties as a sand cast 0.04 Mg nodular iron. The process ofvery rapidly cooling the nodular iron pipe metal through its uppercritical temperature range (1650- l750) and subsequently annealingwould, in a sense, be equal to adding five times more Mg to an iron madeunder the known processes wherein the product is normally sand cast.Stated another way, if one-does not chill cast, he must add greateramounts of Mg to obtain the desired product. If one does chill cast, onthe other hand, less amounts of Mg are required to obtain the desiredproduct having comparatively smaller nodules, and aggregates and flakesas shown, and the residual Mg content must be controlled at less than0.01% or additional annealing ovens or longer annealing ovens will benecessary.

Step VI In Step VI, the cementitic cast pipe are charged into the shortcycle annealing oven. In annealing I seek to effect structural changesin the metal, e. g., cementite transformation to pearlite and graphite;pearlite to ferrite and graphite. To attain the desired transformationannealing must be carried out within a critical temperature range. Inchill casting I have formed cementite by rapid cooling of the cast ironthrough the range of l650 F. to 1750 F.; now on transfer from the chillcasting machine heat is lost. Hence upon annealing the cementiticproduct is heat-treated rst in the zone of l650-1750 F. for cementitetransformation and secondly in the critical range of 1250 to l450 F. forpearlite transformation. As stated the transformations ultimately yieldthe ferrite with graphitic nodules, flakes and aggregates. My annealingcycle is therefore designed so that the nodular iron is raised to l750F. and held at this temperature to transform the cementite to pearlite,ferrite, and free graphite. In the case of nodular iron pipe metal, thehigher the temperature of the first-stage anneal the more rapidly thetransformation of cementite occurs. Beyond 1750 F., excessive oxidationand out-of-roundness is encountered andthe more diicult it would bemechanically to drop the temperature of the pipe to the second-stageannealing zone '(1250-1450" F.) where pearlite is transformed to ferriteand free graphite.

Since this is a process in which metallurgical control features are ofparamount importance, I will now describe in detail the controls used.

CARBON CONTROL The total carbon content of the metal tapped from thebasic cupola will be in the range of 4.00 to 4.60% and can be adjusteddownward to the desired level by an vaddition of cold steel scrap in theelectric furnace. There are possibilities to reduce the carbon contentof the metal as tapped from the basic cupola, by changing thecomposition of the basic slag and thereby reducing the amount of coldsteel scrap to be added in the electric furnace. If, in the process ofrefining the basic slag of the cupola for the most economical electricfurnace operation, the total carbon content is lowered below the desiredlevel, carbon could be added to the bath by the injection process.

CONTROL OF MANGANESE The manganese control of our raw materials issomewhat iixed so that the melt from the basic cupola will containmanganese in the range of 0.35% to 0.55%. A nodular iron containingmanganese in this range will anneal satisfactorily in our annealing ovenwith the short annealing cycle. There is a possibility, however, of somemanganese steel scrap or other scrap materials containing manganesehigher than the indicated desired level, entering into the cupolamixture. The manganese content, in these few instances, may be adjustedto the desired level by an addition in Step II (Figure l) in theelectric furnace of converter metal which would be on hand to take careof this adjustment, or the use of a special low-manganese pig ironpurchased from an outside source.

The converter metal referred to is one low in manganese, silicon andcarbon. The use of the converter metal takes the manganese percentagedown and silicon and carbon can be added if necessary to obtain thedesired percentage of these elements.

CONTROL OF SILICON It is to be expected in this process, that thesilicon content of the metal in the electric furnace would always be onthe low side of the desired range. Silicon can be added in the electricfurnace by an addition of any of the commercial grades of ferro-silicon,or by the injection of silicon metal.

In the extreme case of having the silicon content too high, pigconverter metal or steel scrap could be added. These materials would, inlowering the silicon content, lower the carbon content and carbon may beinjected to raise the level to the desired range.

CONTROL OF PHOSPHORUS The phosphorus content of the nodular iron pipeshould not exceed 0.12%, for best physical properties. If, in the use ofhigh percentages of cast scrap in the cupola charge, the phosphouscontent is above the desired level, steel scrap may be added to theelectric furnace to adjust the phosphorus content to 0.12% maximum. Thesteel scrap addition will lower the total carbon content and the carboncan be adjusted by injection.

In connection with the process and product made by this process, DeLavaud pipe, centrifugally cast in steel molds, and the thin-wallnodular iron pipe made by my process described in this application isdesirable. The regular grey iron super De Lavaud pipe is one with adense close grain metal which is very suitable for ordinary applicationsbut the tests that follow will show that it is in practically every wayinferior to the product made by my process. A comparison between thepipes from the point of view of photomicrographs will indicate radicaldifferences in the structure of the metal. Pipe made by my process havephysical properties which are ample to justify the thin wall thickness Imay employ, for I can have a cona comparison between regular superl 1 1siderably thinner wall pipe with greater strength than with the regularsuper De Lavaud pipe.

CHEMICAL COMPOSITION 1. Thickness is actual thickness of pipe wall.

v 2. Detiection is distance strip bent before breaking.

3. Lbs. load is actual transverse load required to break a y" wide x l2long strip machined from Wall of pipe on centers.

4. M/E is Modulus ot Rupture which is a measure of strength. Pipespecifications require that this be in excess of 40,000 p. s. i.

5. M/R is Modulus of Elasticity which is in this test a measure ofstitlness or resistance to bending. Pipe specifications require thatthis not exceed 12,000,000 p. s. i.

6. B'Rockwell is measure of hardness across pipe wall. Pipe specicationsrequire that this not exceed 95.

RING CRUSHING TESTS [Tests made on 3 length rings by standard procedure]Regular New 6" Pipe Super Process De Lavaud Lbs. Load G, 280 3,860Deflection, inches. .130 .665 Thickness, inches. .418 246 M R 64, 300135, 300

BURSTING TESTS [Tests made on 7%" lengths of plain end pipe. Pipe burstby applying hydrostatic pressure at a slow steady rate] Regular NewSuper Process De Lavaud Minimum Thickness, inches .36S 211 AverageThickness, inches .370 .215 Bursting Pressure, p. s. i 3, 625 3, 200Bursting Tensile, p. s. i 29, 900 63, 500

SUMMARY OF PHYSICAL PROPERTIES Referring to the drawings, Figure 3 is aphotomicrograph of the metallic structure in a regular super De Lavaudpipe 6 inches in diameter cast to 0.38 inch wall thickness andmagnification 100 diameters. The metal here is unetched. Thisphotomicrograph, Figure 3, shows typical size, pattern and distributionof graphite found in the wall of a super De Lavaud pipe. The flakes ofl12 graphite are numerous and extremely small and are arranged in whatis known-as aL-dendritic pattern.

Figure 4 is a photomicrograph of the metallic structure of an iron pipemade by my process. The pipe is 6 inches in diameter with a wallthickness of 0.25 inch. Here also the magnification is diameters and themetallic surface is unetched. The graphite particles throughout the wallof the new process pipe are larger and far less numerous than in Figure3 and generally tend to be somewhat spherical in shape. They are quiteuniform in distribution and show no tendency to the dendritic patternshown in photomicrograph Figure 3.

Referring to Figure 5 I have shown a photomicrograph of the metallicstructurev of a regular super De Lavaud pipe of 6 inches diameter castto 0.38 inch wall thickness; magnification is 500 diameters and the pipehas been etched with a 2% solution of nitric acid and alcohol. Thisphotomicrograph is a portion of the field shown in photomicrographFigure 3 after etching and at a higher magniiication. The structureconsists of a line graphite network in a matrix of ferrite. A smallamount of iron phosphide is visible in the lower portion of the eld.

In comparison, see Figure 6 which is a photomicrograph of a 6 inchdiameter iron pipe made by my process cast to 0.25 inch Wall thickness.The surface of the metal has been etched in the customary manner and themagnication is 500 diameters. 'This is a portion of the field shown inphotomicrograph Figure 4 after etching and at a higher magnication. Thestructure consists of compact particles of graphite in a matrix ofpractically all ferrite. Traces of pearlite and cementite are present.

In conclusion, whereas the iron melted in the basic cupola method had ahigh carbon and low sulphur content, in the acid cupola method, the ironmelted has a low carbon and high sulphur content. In the acid process,it is proposed to eliminate rehearing the melt. It will be recalledthat, in addition to the requirement for heating as in Step II (basiccupola method) it was necessary to adjust, by a steel scrap andferro-silicon addition, the carbon and silicon contents afterdesulphurization. Now by variation of the charge in the acid cupola,such adjustments are unnecessary. As an elucidation of the first step ofthis acid cupola method, attention is directed to the following chart inwhich the precharges of elements of the melt as shown, said elementsbeing tapped from the cupola.

Composition o1 Metal (h) Coke and iux per ton of metal charged into acidcupola:

300# Coke.

50# Silicon Rock. 25# Dolomite.

Sulphur in the iron tapped from the acid .cupola would be reduced indesulphurization (Step I) from 0.10 to 0.02% by an injection of 20# ofcalcium carbide per ton of metal. In acid cupola melting practice thereis a gain in sulphur, phosphorous, total carbon being as shown in thechart. Through this process, I usually obtain the following percentagesof elements in metal tapped from the acid cupola; 2% silicon; 0.45%manganese; 3.40- 3.50% total carbon; 0.l0%.phosphorus; 0.07 to 0.11%sulphur.

It will be apparent from the foregoing that my processes have produced:a valuable iron at reasonable cost 13 with control factors which permitof uniform results in the tinal product.

I desire that my invention be limited solely by the scope of theappended claims and the showing of the prior art.

1. A process of producing an annealed cast iron of irnproved ductilitywhich comprises the steps of forming a molten iron containing a lowphosphorus and normal manganese content, adjusting the carbon, siliconand sulphur content thereof to form a composition which would yield agrey iron in as-cast condition, injecting said molten iron with anintimate admixture of calcium carbide and a magnesium-cerium bearingnodularizing agent to form a composition which would yield a grey ironcontaining free graphite in nodular form in as-cast condition, chillcasting the molten composition to produce a white iron in whichsubstantially all of the carbon remains in combined form, reheating thecast iron to a temperature of from l6501750 F. to convert the ironmatrix from cementite to pearlite and lowering the temperature graduallyto 1250 F. to transform the pearlite to ferrite and to release thecombined carbon as a mixture of nodules, flakes `and aggregates.

2. A process of producing an annealed cast iron of improved ductilitywhich comprises the steps of forming a molten iron containing a lowphosphorous and normal manganese content, adjusting the carbon, siliconand sulphur content thereof to form a composition which would `yield agrey iron in as-cast condition, injecting said molten iron with anintimate admixture of calcium carbide and a magnesium-cerium bearingnodularizing agent in such quantities as to form a compositioncontaining residual magnesium in an amount less than 0.01% by weight andresidual cerium in an amount so slight as to be determinable onlyqualitatively which would yield a grey iron containing free graphite innodular form in as-cast condition, chill casting the molten compositionto produce a white `iron in which substantially all of the carbonremains in combined form, reheating the cast iron to a temperature offrom 1650-l7507 F. to convert the iron matrix from cementite to pearliteand lowering the ternperature gradually to 1250 F. to transform thepearlite to ferrite and to release the combined carbon as a mixture ofnodules, akes and aggregates.

3. A process of producing an annealed cast iron of improved ductilitywhich comprises the steps of forming a molten iron containing a lowphosphorus and normal manganese content, adjusting the sulphur contentof the molteniron to a maximum of 0.02% by weight by the injectiontherein of calcium carbide, adjusting by selective additions of steelscrap and ferro-silicon the carbon content of the molten iron toconstitute approximately 4% by weight and the silicon content of themolten iron to constitute approximately 1.75% by weight of thecomposition which would yield a grey iron in as-cast condition,injecting said molten iron with an intimate admixture of calcium carbideand a magnesium-cerium bearing nodularizing agent in such quantities asto form a composition containing residual magnesium in an amount lessthan 0.01% by weight and residual cerium in an amount so slight as to bedeterminable only qualitatively which would yield a grey iron containingfree graphite in nodular form in as-cast condition, chill casting themolten composition to produce a White iron in which substantially all ofthe carbon remains in combined form, reheating the cast iron to atemperature of from l650-1750 F. to convert the iron matrix fromcementite to pearlite with a partial release of the combined carbon andlowering the temperature gradually to 14 1250 F. to transform thepearlite to ferrite with a further release of combined carbon, the freedgraphite appearing as a mixture of nodules, flakes and aggregates.

4. A process as defined in claim 3 in which the molten iron isinoculated by an addition of ferro-silicon alloy immediately precedingcasting thereof.

5. A process of producing an annealed cast iron of improved ductilitywhich comprises the steps of forming a molten iron with a phosphoruscontent below 0.12% by weight and a manganese content in the range offrom 0.35% to 0.55% by weight, adjusting the sulphur content of themolten iron to a maximum of 0.02% by weight by the injection therein ofcalcium carbide, adjusting by selective additions of steel scrap andferro-silicon alloy the carbon content of the molten iron to constituteapproximately 4% by weight and the silicon content of the molten iron toconstitute approximately 2.75% by Weight of the composition which wouldyield a grey iron in ascast condition, injecting said molten iron withan intimate admixture of calcium carbide and a magnesium-cerium bearingnodularizing agent in such quantities as to form a compositioncontaining residual magnesium in an amount less than 0.01% by weight andresidual cerium in an amount so slight as to be determinable onlyqualitatively which composition would yield a grey iron containing freegraphite in nodular form in as-cast condition, chill casting the moltencomposition to produce a white iron in which substantially all of thecarbon remains in combined form, reheating the cast iron to atemperature of from 1650l750 F. to convert the iron matrix fromcementite to pearlite With a partial release of the combined carbon andlowering the temperature gradually to 1250 F. to transform the pearliteto ferrite with a further release of combined carbon, the freed graphiteappearing as a mixture of nodules, flakes and aggregates.

6. A process as deiined in claim 5 in which the molten iron isinoculated by an addition of ferro-silicon alloy immediately precedingcasting thereof.

7. A process as defined in claim 5 in which the nodularizing agent is analloy of magnesium, cerium and silicon and the ratio of magnesium tocerium therein is at least 4 to 1.

8. A process as dened in claim 5 in which the molten metal is at atemperature of from 2650-2700 F. at the start of the calciumcarbide-nodularizing agent injection.

9. A process as defined in claim 1 in which the nodularizing agentinjection is performed in the absence of oxygen, primarily todesulphurize the metal and secondarily to remove inuential oxides,nitrides, and elemental nitrogen from the metal.

10. The process according to claim 9 in which the injection is performedwith an inert gas as a carrier for the nodularizing agent.

11. The process according to claim l0 in which the gas is nitrogen.

12. The process according to claim 10 in which the gas is carbondioxide.

13. The process according to claim 10 in which the gas is argon.

References Cited in the le of this patent FOREIGN PATENTS France Dec.16, 1955 OTHER REFERENCES going Society.

UNITED STATES PATENT OFFICE Certiicate of Correction Patent No.2,855,336 Octobe17, 1958 Thomas W. Curry It is hereby certified thaterror appears `in the above numbered patent nquiring correction and thatthe said Letters Patent should read as corrected below.

In the drawings, Sheet 3, Fi 4, for (lOOX nital etched) read (DOxunetched')-; Fig. 5, for (500x unetc ed) read (5OOX ni-tal etched)-;colunn 2, line 13, for residum read -residuum; line 56, for additves,read additives-g column 3, line 12, for notal read -nital; line 65, forMB read -ME (modulus of elasticity); column 4, line 16,101' phosphorousread -phosphorus--; column 5,

lines 65 and 66, for

CaO.MgO read CaO.MgO 11203502 AI2O3-SiO2 column 6, line 66, for Step IIread -Step I; column 7, line 5, for Step IV read -Step III-; line 17,for Step II read -Step III-; line 20, strike out (Step V) line 23, forStep II read -Step III-; line 44, for Step IV read Step III; column 9,line 18, for ID read III); column 10, line 57, for phosphous read-phosphorus; column 11, line 30, item 4, for M/E read -M/R-; line 31,tem 5, for M/R read -M/E; column 12, line 68, and column 13, line 27,for phosphorous, each occurrence, read -phosphorus.

Signed and sealed this 31st day of March 1959.

Attestr KARL H. AXLINE, ROBERT C. WATSGN, Attestzng Oycer. Uommissz'onerof Patents.

1. A PROCESS OF PRODUCING AN ANNEALED CAST IRON OF IM-PROVED DUCTILITYWHICH COMPRISES THE STEPS OF FORMING A MOLTEN IRON CONTAINING A LOWPHOSPHORUS AND NORMAL MANGANESE CONTENT, ADJUSTING THE CARBON, SILICONAND SULPHUR CONTENT THEREOF TO FORM A COMPOSITION WHICH WOULD YIELD AGREY IRON IN AS-CAST CONDITION, INJECTING SAID MOLTEN IRON WITH ANINIMATE ADMIXTURE OF CALCIUM CARBIDE AND A MAGNESIUM-CERIUM BEARINGNODULARIZING AGENT TO FORM A COMPOSITION WHICH WOULD YIELD A GREY IRONCONTAINING FREE GRAPHITE IN NODULAR FORM IN AS-CAST CONDITION, CHILLCASTING THE MOLETN COMPOSITION TO PRODUCE A WHITE IRON IN WHICHSUBSTANTIALLY ALL OF THE CARBON REMAINS IN COMBINES FORM, REHEATING THECAST IRON TO A TEMPERATURE OF FROM 1650*-1750*F. TO CONVERT THE IRONMATRIX FROM CEMENTITE TO PEARLITE AND LOWERING THE TEMPERATURE GRADUALLYTO 1250* F. TO TRANSFORM THE PEARLITE TO FERRITE AND TO RELEASE THECOMBINED CARBON AS A MIXTURE OF NODULES, FLAKES AND AGGREGATES.